U.S. patent application number 13/758257 was filed with the patent office on 2013-10-03 for radio frequency antenna circuit.
This patent application is currently assigned to NXP B.V.. The applicant listed for this patent is NXP B.V.. Invention is credited to Liesbeth Gomme, Anthony Kerselaers.
Application Number | 20130257676 13/758257 |
Document ID | / |
Family ID | 45929407 |
Filed Date | 2013-10-03 |
United States Patent
Application |
20130257676 |
Kind Code |
A1 |
Kerselaers; Anthony ; et
al. |
October 3, 2013 |
RADIO FREQUENCY ANTENNA CIRCUIT
Abstract
Presented is radio frequency antenna circuit for portable and/or
compact electronic devices. Embodiments comprise an antenna
connected to an unbalanced current feeding arrangement. The
unbalanced feeding arrangement may generate common mode currents
which increase the overall radiation resistance and efficiency of
the antenna circuit.
Inventors: |
Kerselaers; Anthony;
(Herselt, BE) ; Gomme; Liesbeth; (Anderlecht,
BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NXP B.V. |
Eindhoven |
|
NL |
|
|
Assignee: |
NXP B.V.
Eindhoven
NL
|
Family ID: |
45929407 |
Appl. No.: |
13/758257 |
Filed: |
February 4, 2013 |
Current U.S.
Class: |
343/860 ;
343/850 |
Current CPC
Class: |
H01Q 1/273 20130101;
H01Q 9/285 20130101; H01Q 1/50 20130101 |
Class at
Publication: |
343/860 ;
343/850 |
International
Class: |
H01Q 1/50 20060101
H01Q001/50 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2012 |
EP |
12162378.9 |
Claims
1. A radio frequency antenna circuit for a portable electronic
device comprising: first and second antenna elements; an inductive
element connected between the first and second antenna elements;
and a feed line comprising first and second electrical conductors
connected to the inductive element, wherein the connection
arrangement of the first and second electrical conductors to the
inductive element is asymmetrical.
2. The radio frequency antenna circuit of claim 1, wherein the
first and second electrical conductors are connected towards one
end of the inductive element and at different distances from a
central point of the inductive element.
3. The radio frequency antenna circuit of claim 1, wherein the
first and second antenna elements are arranged parallel to each
other and spaced apart with a dielectric substrate material
provided therebetween.
4. The radio frequency antenna circuit of claim 1, wherein the
first and second antenna elements are adapted to resonate at a
first frequency, and wherein the inductive element and the first
and second antenna elements have a combined resonant frequency at a
second frequency that is different from the first frequency.
5. The radio frequency antenna circuit of claim 4, wherein maximum
dimension of the first and second antenna elements is not more than
half the wavelength of the second frequency.
6. The radio frequency antenna circuit of claim 1, wherein the
first and second antenna elements are adapted to generate a first
common mode current component in the feed line, and wherein the
asymmetrical connection of the first and second electrical
conductors are adapted to generate a second common mode current
component in the feed line.
7. The radio frequency antenna circuit of claim 1, further
comprising: a receiver or transmitter unit; and a matching unit
connected between the receiver or transmitter unit and the feed
line, the matching unit being adapted to substantially match the
impedance of the radio frequency antenna circuit to the impedance
of the receiver or transmitter unit.
8. The radio frequency antenna circuit of claim 1, wherein the
first frequency is substantially equal to the second frequency.
9. A portable electronic device comprising a radio frequency
antenna circuit according to claim 1.
10. The portable electronic device of claim 9, wherein the portable
electronic device is a hearing aid.
Description
[0001] This invention relates to a radio frequency (RF) antenna
circuit for use in a portable electronic device such as a hearing
aid.
[0002] A basic hearing aid typically comprises a microphone,
speaker and associated electronics. In such hearing aids, an
earpiece microphone converts acoustic waves into electrical signals
representing the acoustical waves. The electrical signals are then
amplified, processed and converted back into acoustical waves.
[0003] It is known to provide a remote control function that
controls the amplification and other settings of the earpiece. By
way of example, U.S. Pat. No. 5,721,789 describes a hearing aid
with a remote control function. It has an antenna that is
externally connected to the earpiece of the hearing aid.
[0004] More advanced hearing aids use wireless audio communication
between two earpieces so that there is only one receiver signal.
The method typically used to establish such communication is based
on inductive coupling. A relatively large voltage, which can be 12
volts AC, is applied to a coil which generates a magnetic field.
Within a short range of this first coil, from a few centimeters to
1 meter, the magnetic field can be induced in a second coil. Using
this method, a short range communication link between two earpieces
can be established.
[0005] Radios communicating in this way use magnetic induction (MI)
to establish the wireless link. The MI field is a non-propagating
near field that exhibits very high roll-off behaviour as function
of distance.
[0006] When a communication link has to be established across a
larger range, like more than 1 meter, prior art solutions use a
radio module that works with electromagnetic (EM) waves. EM waves
are able to propagate over large distances and the power rolls off
as the inverse of the square of the distance from the source.
However, it is difficult to implement a radio module in the
earpiece due to size and power-consumption requirements. Known
arrangements therefore implement a radio module in the remote
control unit. In such an arrangement, a first communication is
established between the earpiece and the remote control based on
inductive near field coupling, and a second communication is
established between the remote control unit and further electronic
equipment (like a cellular phone) by means of electromagnetic
radiation. Several hearing aid products based on this concept are
known and available to purchase, of which some employ the
Bluetooth.TM. standard as the second communication protocol.
[0007] The antenna bandwidth represents the frequency range in
which the antenna can be used with sufficient efficiency. For
example, the bandwidth that is required to operate in the worldwide
2.4 GHz ISM band is 84 MHz. It is well-known that antenna bandwidth
is proportional to antenna size.
[0008] Another factor associated with the design of integrated
antennas is the desired input impedance. It is normally preferred
to have a reasonable impedance matching between the antenna and the
RF integrated circuit. Without proper matching, available power
from the RF integrated circuit is not accepted by the antenna and
reflected back to the source. A measure of matching quality can be
expressed by the Return Loss over the operating band.
[0009] Integrating an antenna that suits electromagnetic radiation
in a physically small (i.e. portable) electronic device, such as a
hearing aid, therefore presents various problems. Portable
electronic devices usually have a dedicated design and/or a small
volume. As a result, there may be very little available space for
the antenna.
[0010] It is well known in the art that the antenna volume defines
various antenna parameters. Electrically small antennas are prone
to reduced radiation resistance, efficiency and gain. They are
difficult to match to the RF integrated Circuit due to a fast
changing reactive component of the input impedance.
[0011] According to an aspect of the invention there is provided a
radio frequency antenna circuit according to the independent
claims.
[0012] Proposed is an antenna arrangement for portable and/or
compact electronic devices, such as a hearing aid, that addresses
various problems associated with integrated antennas and offers a
sufficient wideband communication channel.
[0013] The antenna may be connected to an unbalanced feeding
arrangement and a radiating feed line. Such an unbalanced feeding
arrangement generates common mode currents in the radiating feed
line. In this way, the radiation efficiency may be increased in a
small volume.
[0014] According to an aspect of the invention there is provided a
radio frequency antenna circuit for a portable electronic device
comprising: first and second antenna elements; an inductive element
connected between the first and second antenna elements; and a feed
line comprising first and second electrical conductors connected to
the inductive element, wherein the connection arrangement of the
first and second electrical conductors to the inductive element is
asymmetrical.
[0015] Embodiments may be directed to the use of hearing aid
systems as wireless communication devices and in particular to high
quality audio communication. High quality audio may be understood
to be CD-like quality sound having a larger audio bandwidth than
voice audio.
[0016] Embodiments may operate in the Radio Frequency (RF) bands by
means of electromagnetic waves and comprise different components
including: an electrically small antenna, an unbalanced feeding
structure, a radiating feeding line, and a matching unit close to
the receiver and transmitter.
[0017] An electrically small RF antenna with unbalanced feeding
arrangement is therefore proposed that may be used to generate an
electrical field radiation pattern that is perpendicular to the
side of a human head.
[0018] According to another aspect of the invention there is
provided a portable electronic device comprising a RF antenna
circuit according to the invention.
[0019] Examples of the invention will now be described in detail
with reference to the accompanying drawings, in which:
[0020] FIG. 1 is a block diagram of a RF antenna system according
to an embodiment of the invention;
[0021] FIGS. 2A and 2B illustrate the generation of a common mode
current within a coaxial cable;
[0022] FIG. 3 is a diagram showing an example of an unbalanced feed
and balanced antenna according to an embodiment of the
invention;
[0023] FIG. 4 shows an exemplary embodiment of an electrically
small radiating element with an unbalanced feeding arrangement;
[0024] FIG. 5 is a block diagram of a RF antenna circuit according
to an embodiment of the invention;
[0025] FIG. 6 illustrates a first implementation example of a RF
antenna circuit according to an embodiment;
[0026] FIG. 7 is a side view of the implementation example of FIG.
6;
[0027] FIG. 8 is an illustration of a simulation model of the
exemplary embodiment of FIGS. 6 and 7;
[0028] FIG. 9 is a graph showing the simulated return loss at the
unbalanced feeding connections of the simulated model of FIG.
8;
[0029] FIG. 10 illustrates the 3-dimensional radiation pattern of
simulation model of FIG. 8; and
[0030] FIG. 11 shows an alternative example of an antenna and
feeding structure according to an embodiment.
[0031] Embodiments relate to an antenna system for small portable
electronic products like hearing aids. The antenna system operates
in the RF band with electromagnetic radiation and is suitable for
integration in physically small electronic devices such as a
hearing aid. Further, it is possible that other communication
systems simultaneously operate in the device, such as a MI
communication system for example.
[0032] Typically, the physical volume of a hearing aid is small
when compared with the required wavelength of operation. For
example, behind the ear (BTE) hearing aids have typical dimensions
of 30.times.12.times.8 mm (and smaller ones have a size of
20.times.14.times.6 mm), whereas the wavelength of the world wide
ISM 2.5 GHz band is 12 cm.
[0033] The dipole antenna is a popular antenna. Such an antenna
requires a total length of a half wavelength, which is therefore 6
cm in the case of an operating frequency of 2.5 GHz.
[0034] Another popular antenna is a monopole antenna. Such an
antenna consists of a quarter wave radiator 3 cm and a ground plane
with a size of at least a half wavelength in one direction 6 cm.
Such antennas are therefore difficult to integrate in small
portable products (like hearing aids) having physical dimensions
smaller than the required antenna size.
[0035] An antenna system according to a proposed embodiment
comprises the following components: a small antenna; an unbalanced
feeding structure; a radiating feeding line; and a matching unit
close to the receiver and transmitter.
[0036] FIG. 1 is a schematic diagram of such an antenna system 100
having a communications section 200 and a radiation structure 300.
The RF port of the receiver or transmitter 10 is connected to a
matching unit 20. The distance between both is short. The RF port
of the receiver or transmitter 10 can be balanced. The matching
unit 20 adapts the input impedance of the radiation structure 300
to the impedance of the RF port of the receiver or transmitter. The
matching unit 20 is connected to a radiating feed line 30 which is
further coupled to an unbalanced feeding structure 40. The feeding
structure 40 is connected to an antenna 50.
[0037] Such an antenna system 100 provides an increased efficiency
due to providing the ability to generate increased common mode
currents in the radiating feed line 30 without requiring an
increase of the physical volume of the antenna 50. Further, the
radiation pattern can be improved in the sense that more radiation
is taking place in different directions when the physically small
antenna 50 and the radiating feeding line 30 are positioned in
different orientations.
[0038] In FIG. 2A, the differential mode (Id) current can be seen
on a coaxial cable 60. Currents flow at the outer side of
conductors for radio frequencies due to the skin effect. For
example, at a frequency of 2.5 GHz, the skin depth in a copper
conductor is 1.3 .mu.m. This is much less than the thickness of
practical conductors. The differential mode current Id flows on the
outer side of the inner conductor to the load L and returns at the
inner side of the outer conductor to the source S.
[0039] In FIG. 2B, the differential mode currents Id flow like in
FIG. 2A. However, due to coupling to a nearby object 65 that
radiates electromagnetic energy or carries RF current, a common
mode current Ic is generated in the outer side of the outer
conductor of the coaxial cable 60.
[0040] Similar effects can occur on balanced feeding lines. The
common mode current Ic flows in only one direction, which is in
contrast with the differential mode currents Id.
[0041] The differential mode currents Id generate magnetic fields
that have an opposite direction and thus cancel each other and no
radiation takes place. However, the common mode current Ic
generates a magnetic field that is not cancelled and radiation
takes place. There is thus a radiation resistance increase due to
common mode currents Ic flowing through the feeding line, wherein
radiation resistance equals radiated power divided by current
squared.
[0042] It will therefore be appreciated that an unbalanced feeding
system in combination with an antenna can generate common mode
currents Ic on the feeding line.
[0043] FIG. 3 shows an example of unbalanced feeding configuration
connected to a balanced half wave dipole antenna 70. The current I1
is different from current I2 due to the unbalanced feeding
arrangement. By applying a voltage source with a frequency tuned to
the dipole antenna 70 to the feeding section, a current I1 is
generated that is lower than I2 since a coil 75 is in series with
the quarter wave antenna element. Part of the current of I2 will be
flowing into the feeding line as common mode current.
[0044] It has been found that the combination of a physically small
antenna close to a nearby object combined with an unbalanced
feeding structure generates even stronger common mode current Ic on
the feeding line, as illustrated in FIG. 4. This is because the
following two different mechanisms generate common mode current:
(i) the unbalanced feeding structure generates different currents
on the antenna, and (ii) the unequal amount of coupling of both
small antenna elements to the nearby object generates a common mode
current.
[0045] The coupling to the nearby object can be seen as unbalanced
capacitance coupling from which a common mode current component is
generated. The common mode current Ic on the feeding line together
with its physical size and shape of the antenna increases the
overall radiation resistance and efficiency of the antenna
system.
[0046] FIG. 4 shows a physically small antenna combined with an
unbalanced feeding structure. First 80 and second 82 antenna
elements are each adapted to resonate at a frequency which is not
within the frequency band of interest. They are resonated with the
inductive coil element 85 that is connected between the antenna
elements 80 and 82. For example, a small antenna element with an
input impedance of 5 pF in series with 10 ohms can be resonated
with a coil feeding structure of 0.8 nH at a frequency of 2.5
GHz.
[0047] To generate larger common mode currents Ic, the feeding is
done by means of connecting to the coil 85 in an asymmetric way. In
other words, the first and second connections of the feed line are
connected to the coil 85 asymmetrically about a central axis of the
coil 85. Thus, the first connection of the feed line is connected
to the coil at a first point, and the second connection of the feed
line is connected to the coil at a second point, wherein the first
and second points are not equidistant from a central point of the
coil.
[0048] At resonance, the voltage at the small antenna element is
multiplied with the quality factor of the resonance circuit. This
results in an increased common mode current Ic since it can be seen
as: antenna voltage/effective coupling impedance.
[0049] In a second embodiment, an antenna element and unbalanced
feeding structure will be explained by reference to different
drawings.
[0050] FIG. 5 shows a diagram of an antenna element and unbalanced
feeding structure according to an embodiment. First and second
antenna elements 11 and 12 are capacitively coupled and do not
resonate at the frequency band of interest. The input impedance is
capacitive with a series resistance. The resistance is composed of
the radiation resistance together with the loss of the antenna
elements.
[0051] An inductance 13, 14 is connected between the antenna
elements 11, 12 and arranged to compensate for the capacitance
formed by the two antenna elements 11,12. The feed line connections
15 are unbalanced and connected to the inductance 13,14 so that the
structure generates common mode currents. In other words, the two
feed line connections 15 are connected to the inductance
asymmetrically, such that the inductance is split into first 13 and
second 14 inductances of differing size.
[0052] The two antenna elements 11, 12 have different coupling
impedances to a nearby object due to their differing distance from
the nearby object N. This can be other conductors in the hearing
aid like the ground reference and the feeding line (not shown on
FIG. 5). As has been explained above, the unequal amount of
coupling of both small antenna elements to the nearby object
generates a common mode current component, thus resulting in
amplification of the common mode current Ic on the feed line
connection.
[0053] FIG. 6 shows a first implementation example of an antenna
and feed arrangement according to an embodiment. First 11 and
second 12 antenna elements are formed from a conductive material,
for example a thin copper sheet. The antenna elements 11, 12 are
separated by means of a dielectric substrate material 16. This can
be air or other low loss dielectric material. The two antenna
elements together with the dielectric substrate are adapted to not
resonant at the required frequency of interest.
[0054] On one side of the dielectric substrate material 16, there
is a distributed inductance 13 between the first 11 and second 12
antenna elements. The inductance 13 together with the antenna
elements 11,12 and the substrate are adapted to resonate at the
required frequency of interest.
[0055] The first 15a and second 15b feed line connections are
connected to the inductance 13 asymmetrically so that the feeding
arrangement is unbalanced and the structure generates common mode
currents. In other words, the two feed line connections 15a and 15b
are connected to the inductance 13 at different distances from a
central axis of the inductance 13.
[0056] The unbalanced connection of the two feed line connections
15a,15b to the inductance 13 can be seen on the side of the
inductance closest to the second antenna element 12.
[0057] Exemplary dimensions of such a structure for operation at
2.5 GHz (i.e. where the frequency of interest is 2.5 GHz) may be as
follows: [0058] Antenna elements: 8.times.12 mm, copper material of
0.1 mm thickness. [0059] An air substrate with 4 mm separation
between the two antenna elements. [0060] The inductor and
unbalanced feed are constructed by means of copper conductors of 35
.mu.meter thickness on printed circuit board material, for example
Rogers 4003.
[0061] FIG. 7 shows the details of the inductance and feeding means
of the exemplary embodiment of FIG. 6. The conductive part may be
varied (as indicated by the arrow labelled "17") tune the resonant
frequency to the required value. It has been found that changing
the position of the conductive part 17 does not change the input
impedance seen at the unbalanced feeding connections.
[0062] Also, the input impedance can be changed by varying the
position of the feeding connections 15a and 15b, as indicated by
the arrow labelled "18".
[0063] FIG. 8 is an illustration of a simulation model of the
exemplary embodiment of FIGS. 6 and 7. More specifically, FIG. 8
shows the 3-dimensional structure that is used for simulation using
an industry-leading 3-dimensional electromagnetic simulator (CST
Microwave studio) from Computer Simulation technologies.
[0064] FIG. 9 is a graph showing the simulated return loss at the
unbalanced feeding connections of the simulated model of FIG. 8.
From this, it can be seen that the combined structure (of the
antenna elements, inductance element and feed line connections)
resonates at a frequency of 2.48 GHz. FIG. 10 shows the
3-dimensional radiation pattern of this embodiment. It can be noted
that if the antenna is placed with the antenna elements parallel to
the X-Y plane, an electrical field radiation pattern is generated
that is elongated in the in the X-Y plane.
[0065] Thus, when an embodiment of the proposed antenna arrangement
is placed close to a human head (in a hearing air for example), two
different electromagnetic propagation modes can be used (so called,
off-body communication mode and on body communication mode).
[0066] The off body communication mode may be, for example,
wireless communication between the hearing aid and a cellular
phone. The on-body communication mode may be, for example, wireless
communication between the hearing aid of each ear.
[0067] It may be preferable that the off-body communication mode
has an electrical field radiation pattern that is mainly parallel
with the plane of the substantially vertical side of the user's
head, whereas it may be preferable that the on-body communication
has an electrical field radiation pattern that is mainly
perpendicular to vertical side of the user's head (so that is
elongated in the same direction as the separation between the
user's ears).
[0068] Ear-to-ear communication may be accomplished with a monopole
antenna perpendicular to vertical side of the user's head. However,
since a typical hearing aid is no larger than 6 mm height this is
not feasible.
[0069] The proposed antenna arrangement, however, can be of reduced
size compared to prior art antenna arrangements whilst providing a
similar radiation pattern. Embodiments are therefore advantageous
for integration into physically small (i.e. compact) electronic
devices such as a hearing aid.
[0070] FIG. 11 shows an alternative embodiment of an antenna and
feeding structure. Here, first 11 and 12 second antenna elements
are circular electrically conducting planar structures adapted to
not resonate in a frequency band of interest.
[0071] The first 11 and 12 second antenna elements are arranged
parallel to each other and space apart with a dielectric substrate
material 16 positioned therebetween.
[0072] Connected between the first 11 and 12 second antenna
elements is an inductive element 13.
[0073] The input impedance is capacitive with a series resistance.
The resistance is composed of the radiation resistance together
with the loss of the antenna elements. The distributed inductance
13 thus compensates for the capacitance formed by the two antenna
elements 11, 12.
[0074] The two feed line connections 15a and 15b are connected to
the inductive element 13 in an unbalanced way so that so that the
structure generates common mode currents. In other words, the two
feed line connections 15a and 15b are connected towards one end of
the inductive element 13 and at different distances from a central
point of the inductive element 13. It will be understood that this
connection arrangement can be described as asymmetrical since the
two feed line connections 15a and 15b are not connected on opposite
sides of a central axis with equal spacing from the central axis
(i.e. the two feed line connections 15a and 15b are not connected
in a symmetrical arrangement).
[0075] The first 11 and second 12 antenna elements have different
coupling impedances to a nearby object, which can be other
conductors in the hearing aid like the ground reference and the
feeding line (not shown on FIG. 11). This results in amplification
of the common mode current on the feeding line and thus increases
the radiation efficiency.
[0076] Embodiments employ two different concepts for generating
common mode current. Firstly, the unbalanced (i.e. asymmetrical)
feeding connection of the feed lines to the inductive element
generates different currents on the antenna, thus generating a
first common mode current component. Secondly, unequal coupling of
the first and second antenna elements to a nearby object generates
a second common mode current component. The combination of these
common mode current components thus provides a stronger common mode
current Ic on the feeding line.
[0077] Generation of a larger common mode current on the feeding
line together with its physical size and shape increases the
overall radiation resistance and efficiency of the antenna
arrangement.
[0078] Various modifications will be apparent to those skilled in
the art.
* * * * *